I know it's premature and it's also rhetorical, in the sense that no team has a shot at slowing him. But in most cases, the Spurs are well equipped to at least adequately defend the oppositions stars. He's the most glaring exception to that I can think of and not just because he's the best player in the league.
Leonard is really their only legit James defender, but even he won't be able to guard him in the post and even if he could, he can't be expected to guard him for roughly 45 mpg and stay out of foul trouble. They clearly need a second guy; unfortunately, they just don't have it. Diaw is too slow (yes, he did a nice job in the first meeting, but James and the Heat in general were clearly coasting once they found out who wasn't playing and didn't bother to start playing until they were in serious danger of losing) to guard him off the dribble and Green isn't big or strong enough. The only other possibilities are Ginobili, who's also not big or strong enough or McGrady, who for obvious reasons probably won't play a meaningful second unless something has gone horribly wrong. He's also a sub par defender that, for all intents and purposes, hasn't really played a game in a few months and that was in China.
This is not the Lakers or Grizzlies, where the Spurs can get away with fronting the post and mostly disregarding their outside shooters. This team is too intelligent to not figure that out and has way too many good shooters, who are due for progression to the mean. They might just have to essentially concede something like 35 per game to him, stay home on their myriad shooters and hope that they can do a solid job on everyone else.